The present invention relates to signal synchronization, and more particularly to signal synchronization in a communications system. Even more particularly, the present invention relates to synchronization of wireless transceivers by transmitting a composite waveform having a known frequency variation to an unsynchronized wireless transceiver of a mobile satellite communications system for signal synchronization and a corresponding method of acquiring the composite waveform at the unsynchronized transceiver.
In mobile communications systems, such as a mobile satellite communications system, mobile communications terminals (i.e. mobile terminals or transceivers) are frequently placed in positions in which they cannot maintain synchronization with a satellite. For example, the mobile terminal might be turned off and stored, may be carried into a building with significant signal attenuation, or may be newly purchased. In any case, the mobile terminal must be able to find the signals sent from a satellite for synchronization prior to being able to begin communication. As is typically the case, a terrestrial gateway station or base station is constantly transmitting bursts containing waveforms used for synchronization over a specified channel, such as a frequency control channel (hereinafter referred to as FCCH). The mobile terminal must find the signal waveform that is transmitted from the gateway station via the satellite in order to be able to begin communication with the gateway station via the satellite. This process is commonly referred to as acquisition.
The gateway station will transmit a synchronization waveform (also referred to as a waveform or signal waveform) as a limited duration burst, e.g. 5 msec, periodically, e.g. once every 320 msec. In acquiring the synchronization signal, a mobile terminal will typically have to search within about one thousand communications channels (i.e. channels) of a communications link to find the waveform over the period of 320 msec for the 5 msec waveform. Often, to save processing during acquisition at the mobile terminal, the mobile terminal will look at approximately ten preassigned channels of the one thousand channels for the transmitted waveform. The mobile terminal must search in different channels because the burst containing the waveform does not usually arrive at the same frequency that the waveform is transmitted at due to frequency shifts/offsets from the gateway station to the satellite and from the satellite to the mobile terminal. Furthermore, the mobile terminal does not know at what time within any given 320 msec window the 5 msec waveform will arrive.
In the prior art, such waveforms are detectable at the mobile terminals. A requirement of such waveforms is that a signal frequency (or carrier frequency) of the waveform be known. However, because of signal degradation and distortions due to all forms of time delays, propagation delays, Doppler shifting, and noise, it is necessary to compensate for the frequency and/or time shifting of the waveform.
By way of background, time delays result from several factors. Time delays may be caused by obstructions in a dominant signal path that generate xe2x80x9cmulti-pathsxe2x80x9d to occur from scattered paths resulting from reflections off the obstructions. The differences in distance between the multi-paths result in timing offsets as well as fading of the signal, depending upon the type of channel (e.g. Rician or Rayleigh fading). Also, in mobile satellite communications systems, in particular, relative motion between the satellite and the mobile terminal due to a velocity of the satellite as it orbits the Earth and another velocity of the terminal as it is operated from a moving vehicle, for example, results in varying time delay by the signals traveling between the satellite and the mobile terminal. Even a stationary mobile terminal may experience relative motion between itself and the satellite due to motion of the satellite in orbit. For example, in a geostationary satellite system, the satellite follows an approximately sinusoidal pattern of north-south movement in orbit. As such, if the satellite and the mobile terminal are moving towards each other, a transmitted signal from the satellite will arrive earlier and earlier as the relative movement continues.
Doppler shifting is also the result of such relative movement of the satellite and the mobile terminal. As the mobile terminal and the satellite move toward one another, frequencies appear to get higher, but as the mobile terminal and the satellite move away from one another, frequencies appear to get lower.
This gives rise to a need for a new type of signal waveform used for synchronization that can easily be detected and be resolved for Doppler shifting (i.e. frequency offsets) and time delays.
Furthermore, in the prior art, different types of signal waveforms are transmitted from the gateway station to a mobile terminals via satellites for synchronization. This is because in satellite communications systems, many different services are provided that operate at disparate bandwidths and tolerate disparate levels of signal attenuation and minimum signal-to-noise ratios (SNRs). Consequently, current mobile satellite systems require more than one type of waveform to be used for the many different services.
As an example, one key service provided by the mobile satellite systems is voice or data communication. Effective voice communication requires that channel attenuations be less than an order of 10 dB whereas other services, such as alerting, may only require that channel attenuations be less than an order of 30 dB. By way of example, if a tree were blocking the line-of-sight path of a voice signal between a satellite and the terminal, and attenuated a voice signal from the satellite by less than 10 dB, the voice signal could still be received by the mobile terminal. However, if a building were blocking the line-of-sight path of the voice signal and attenuated the voice signal by 20 dB, then the mobile terminal could not receive the voice signal but could still track and receive an alerting signal from the satellite.
In contrast, in xe2x80x9calertingxe2x80x9d, the mobile satellite system typically tolerates very low signal conditions as compared with conditions that can be tolerated by a typical voice signal. Specifically, there is typically a 20 dB difference in channel attenuation levels between conditions in which an alert signal can be successfully received and conditions in which a voice signal can be successfully received. Thus, conditions supporting alert signals will not necessarily support voice signals in the same mobile satellite system.
Thus, in order to accommodate the use of such varying signal level tolerances (e.g. voice services and alerting services) the prior art utilizes distinct waveforms for each distinct service, e.g., one waveform for tracking alerting, and another waveform for synchronizing voice communications.
Thus, there is a need in the wireless communication industry to provide a waveform that supports a variety of services under different signal conditions to conserve power and resources in the mobile satellite system.
One example of a prior art waveform which is easily detectable and resolvable for frequency shifting, but not timing offsets, and which has been used in prior art communications systems is a sinusoidal waveform, or a xe2x80x9ctonexe2x80x9d, which has a constant frequency over time. An example of such a tone is a sine wave or cosine wave with any random phase xcfx86.
Although sinusoidal waveforms or tones are easily detectable by mobile terminals, they present a problem of xe2x80x9cspursxe2x80x9d in the received frequency when the tone travels in mobile terminal""s hardware. The receiver hardware of the mobile terminal typically sees a received waveform (i.e. tone) as a very low level signal. Furthermore, a receiver typically includes a variety of frequency sources that tend to induce sinusoids to the received signal (i.e. tone), making distinctions between the received signal and the induced sinusoids difficult to discern.
These induced sinusoids are called xe2x80x9cspursxe2x80x9d which are essentially frequency-domain spikes. When spurs occur in the receiver, there can be a loss of synchronization because the waveform of interest (i.e. the tone) may be lost. One way to avoid these spurs is to change RF hardware so that they are eliminated by the hardware, however this is costly and takes space and power within the hardware itself.
Thus, another desirable design constraint of the waveform (which may be referred to as a synchronization signal) is that it not require a complex receiver or complicated changes to standard receiver hardware and that the waveform not induce hardware related degradation thereof. In particular, the waveform must be robust against induced spurs.
Other prior art methods have eliminated the problem of xe2x80x9cspursxe2x80x9d by implicitly spreading the spurs by using a signal waveform such as a Quadrature Phase Shift Keyed (QPSK) signal. Such a QPSK signal is modulated according to phase and therefore is not correlated to the spurs. Although the QPSK modulated signal solves the problem of spurs, it does not solve the detectability of varying signal levels used in multiple levels of services, such as voice and alerting services.
Other prior art methods have used, as the waveform used for synchronization, a tone followed by a modulated waveform, wherein the tone is used to compute frequency and the modulated waveform is used to compute timing. Although the use of the tone followed by the modulated waveform solves the problem caused by induced spurs, it exhausts system resources such as power, bandwidth and time, and is not well-suited to two or more levels of service, such as alerting and voice communication.
With regard to the acquisition of the waveform at a mobile terminal, and considering several of the above stated concerns it is desirable to transmit a waveform that is easily detectable, can resolve both frequency and timing offsets, avoids the problem-of xe2x80x9cspursxe2x80x9d in the receiver, and can support different types of services, e.g. acquisition of voice services and tracking of alerting. Thus, such a waveform should be practical to implement on a digital signal processor, for example, using a Fast Fourier Transform (FFT), should not resemble a tone, and should not require hardware modifications in the receiver.
The present invention advantageously addresses the above and other needs.
The present invention advantageously addresses the needs above as well as other needs by providing a method and apparatus for enabling frequency offset and time offset estimation such as for synchronization of a wireless communications terminal that is easily detectable, can resolve both frequency and timing offsets, mitigates xe2x80x9cspursxe2x80x9d, can support different types of services, and may be implemented on a digital signal processor.
In one embodiment, the present invention may be characterized as a waveform to be transmitted as a burst within a channel that is used for the synchronization of unsynchronized wireless communications terminals in a wireless communications system that consists of a composite waveform. The composite waveform comprises two or more component waveforms, wherein each of the two or more component waveforms has a known frequency variation throughout the burst.
In a preferred embodiment, the composite waveform has a composite bandwidth on an order of an available channel bandwidth and each of said two or more component waveforms have a component bandwidth on the order of the available channel bandwidth. Furthermore, a range of values for the differences between the instantaneous frequencies of two of said two or more component waveforms is on an order of twice of said available channel bandwidth.
In another embodiment of the present invention, the composite waveform comprises a dual-chirp waveform including an up-chirp component waveform and a down-chirp component waveform.
In another embodiment, the present invention may be characterized as a method of facilitating waveform synchronization in a wireless communications system, the method comprising the step of transmitting a burst comprising a composite waveform comprising two or more component waveforms on a channel, wherein each of the two or more component waveforms has a known frequency variation throughout the burst.